In certain vehicles, such as tractors or the like, steering is accomplished by driving. Skid steer loaders steer by driving, but typically accomplish this steering with hydrostatic power units. Other vehicles accomplish steering by differential driving. For example, the Van der Lely, U.S. Pat. No. 4,273,206, discloses a pair of torque converters for the rear wheel drive of a tractor, in which each output for the ground engaging units is transmitted through a planetary gear system. Others use a conventional clutch system for selectively driving one or the other side of the vehicle.